Sysadmins and anyone else writing administrative scripts should be intimately familiar with the following system directories.
Binary executables. Basic system programs and utilities (such as bash).
More system executables.
Superuser binaries. Basic system administrative programs and utilities (such as fsck).
More superuser binaries.
Et cetera. Systemwide configuration scripts.
Boot scripts, on Red Hat and derivative distributions of Linux.
Documentation for installed packages.
System temporary files.
Systemwide log files.
User mail spool.
Some early Unix systems had a fast, small-capacity fixed disk (containing /, the root partition), and a second drive which was larger but slower (containing /usr and other partitions). The most frequently used programs and utilities therefore resided on the small-but-fast drive, in /bin, and the others on the slower drive, in /usr/bin.
This likewise accounts for the split between /sbin and /usr/sbin, /lib and /usr/lib, etc.